Absorption spectroscopy is a common laser diagnostic technique for providing quantitative thermodynamic data in industrial systems with minimal optical access [1]}. In absorption spectroscopy, laser light at a frequency that is resonant with a quantum energy-level transition within an atom or molecule propagates through an unknown environment to a detector (Figure REF a). The amount of laser light that the target species absorbs along the path is directly proportional to the species concentration, and nonlinearly related to the temperature of the system. The nonlinear response to temperature depends on the lower state energy (\(E”\) ) of the quantum transition, as shown in Figure REF b. Therefore, a ratio of the absorption on two transitions with different \(E”\)  cancels the dependence on concentration and results in a monotonic function of temperature that can be used to measure temperature in a system. This so-called “two-line” thermometry technique is the most common form of absorption thermometry for gases [2]}.
